demo: maxmin filter examples

[johnnychen94]

Took a quick look at [1] and I feel this is worth a demonstration card.

References:

[1] Verbeek, P. W., Vrooman, H. A., & Van Vliet, L. J. (1988). Low-level image processing by max-min filters. Signal Processing, 15(3), 249-258.

[timholy]

Pretty cool. And mapwindow is really efficient thanks to the Lemire filters.

[Dsantra92]

Can I work on this ?

[johnnychen94]

Yes, of course!

We are plan to host hacktoberfest locally in JuliaCN during Oct 17-18, so if you're going to work on this, please submit an early PR to notify other potential participants.

[Dsantra92]

@johnnychen94 is a pr within this week is okay?

[Dsantra92]

@johnnychen94 @timholy just a few set of questions.

1. Firstly I found 2 ways to find minmax, one was to use extrema and other was to use maximum on top of minimum.Both using mapwindow.I found extrema to be very efficient but I had to go with minimum+maximum because extracting min or max filtered images of extrema also incurrs some cost, or am I missing an easy way to do so ?

2. I have done the maxmin, minmax, min and max filters.Are there any more implementations you would like to add ?

[johnnychen94]

I'm surprised that extrema even works. The performance difference might be explained because extrema is written in a more efficient way while maximum is actually implemented as mapreduce(identity, max, img). You could use @which maximum(patch) to check where it's defined.

I suggest still using minimum and maximum in the demo because this performance difference, if needed, should be fixed upstream.

FYI, mapwindow! can be used to reduce additional memory allocation when you have successive filters.

I have done the maxmin, minmax, min and max filters.Are there any more implementations you would like to add ?

I don't have specific ideas in mind, if you have good ideas, feel free to add them.

I haven't check it yet, https://github.com/sairus7/MaxMinFilters.jl sounds like a promising functionality to be added into ImageFiltering.

[johnnychen94]

Added in #190

[timholy]

In this case extrema is just being used for dispatch---it's not really running extrema per se. The point is that the Lemire algorithm requires that both the min and max be extracted---it's integral to the computation itself.

You can easily "extract" one or the other this way:

minmax = mapwindow(extrema, img, window)
imgmin = map(first, minmax)
imgmax = map(last, minmax)

You could alternatively use MappedArrays and do the latter "lazily" (which would be especially nice for big data).

[johnnychen94]

So obviously a fact that I did not realize! I thought without a second concluded that two map(first, ) is more expensive than mapwindow(maximum, ) :joy: I should really benchmark things.